A popular holiday destination, the Lake District in northwestern England is a region of picturesque mountains and long, narrow lakes. Most of the lakes lie in U-shaped valleys that were carved by glaciers during the last ice age. Morecambe Bay, below the Lake District, opens into the Irish Sea. This large expanse of intertidal sand and mudflats is notorious for its quicksand and tides that are said to move "as fast as a horse can run."